1. Field of the Invention
This invention relates to a toner for development of electrical latent images or magnetic latent images in electrophotography, electrostatic printing, etc. More particularly, the present invention relates to a heat-fixable dry system toner excellent in lower temperature fixability in a hot roller fixing system.
2. Description of the Prior Art
In the prior art, there have been known a number of electrophotographic methods as disclosed in U.S. Pat. Nos. 2,297,691, 3,666,363 and 4,071,361. Generally speaking, the electrophotographic method comprises forming electrical latent images on a photosensitive member by utilization of a photoconductive material according to various means, then developing said latent images by use of a toner, optionally transferring the toner images onto a transfer material such as paper, and thereafter fixing the developed images by heating, pressure or solvent vapor to obtain copied products.
As the method for development to visualize the electrical latent images by use of a toner, there have been known the powder cloud method as disclosed in U.S. Pat. No. 2,221,776, the cascade developing method as disclosed in U.S. Pat. No. 2,618,522, the magnetic brush method as disclosed in U.S. Pat. No. 2,874,063, and the method in which the electroconductive magnetic toner is employed as disclosed in U.S. Pat. No. 3,909,258.
The toner employed in development is generally prepared by mixing and dispersing a colorant into a thermoplastic resin, followed by micropulverization. As the thermoplastic resin, there have been widely used polystyrene resins, polyester resins, epoxy resins, acrylic resins, urethane resins or copolymer resins thereof. As the colorant, carbon black is used most widely, and black magnetic powders of iron oxide type are frequently used in case of magnetic toner.
As the system for fixing of a toner onto papers or other materials, there have been developed various methods and devices. Among them, frequently employed are systems by use of heat, as exemplified by the oven fixing system. Whereas, with the trend for minituarization and speed-up of copying machine under the policy to make copying operations more efficient and energy-saving in recent years, the hot roller fixing system which is a pressure contact heating system advantageous in heat efficiency is becoming most popular. Such a system brings the toner image on a paper to be pressure contacted directly with a hot roller, and it is very advantageous in heat efficiency and also most suited for speed-up from the standpoint of fixability of toner onto a paper. On the other hand, such a system inherently involves serious problems. Shortly speaking, when the hot roller is maintained at a temperature at which the toner can be fixed, the toner is adhered not only onto the paper but also onto the hot roller surface, until the non-image area on the paper is contaminated after repeated copying, thus exhibiting the so called off-set phenomenon.
For overcoming or alleviating such problems, various countermeasures have been sought for in fixers and toners, but no satisfactory solution has yet been obtained.
So far as the fixer is concerned, there have been made contrivances such as coating the hot roller surface with an excellently releasing material such as Teflon, silicone, etc. simultaneously with coating of the roller surface with a releasing oil such as silicone oil also for the purpose of fatigue prevention. However, such an oil coating involves unfavorable problems in practical application such as complication of the fixer brought about by provision of an oil coating system as well as its cost.
With regard to the toner, it has been practiced to add waxes such as low molecular weight polyethylene, polypropylene, etc. for the purpose of increasing releasability thereof. However, for sufficient prevention of off-set, such a material is required to be added in a considerably large amount, which will result in deleterious effects such as increased agglomerating tendency, lowering of durability due to worsening of free flow property and instabilization of charging characteristics, etc.
As another method, improvements of binder resins have been considered. For example, U.S. Pat. No. 3,941,898 proposes a toner in which a crosslinked vinyl type polymer is used as the binder resin. According to this method, while marked improvements can be obtained in off-set resistance and entanglement resistance, there ensues the drawback that the increase of crosslinking degree will result in disadvantageous elevation of the fixing point. Also, the crosslinked vinyl type polymer suffers from the drawback that the toner using the crosslinked vinyl type polymer as the binder resin can give no good developing characteristic, because pigments can difficultly be dispersed therein or it is poorly compatible with other polymers. In particular, in case of a magnetic toner, there are many problems, because fixing temperature is elevated to worsen off-set resistance. To describe in more detail, for example, styrene-butyl acrylate is crosslinked with divinylbenzene to prepare various crosslinked resins having different degrees of crosslinking. Toners are prepared by use of these resins, and fixing characteristics and off-set resistances thereof are examined. Off-set is clearly difficulty caused as the crosslinking degree is higher, but contrariwise the fixing temperature is increased. By comparison between the crosslinked polymer and the non-crosslinked polymer, the former is clearly broadened in non-off-set fixing temperature range but it is not at a practically sufficient level for a fixing roller which is lowered through fatigue in releasability on the surface.
Moreover, for attempting a high speed fixing, the resin must inevitably be lowered in its molecular weight to have a lowered softening point, as contradictory to the measure for improvement of off-set resistance. This also necessarily leads to lowering of the glass transition point of the resin, whereby unfavorable phenomenon of blocking of toner during storage may be caused. In low-speed or medium-speed copying machines of the prior art, for which low temperature fixing is not required so much, these troubles were no problem at practical levels by employment of countermeasures as described above such as improvement of the fixer or other measures in the process, use of crosslinking, releasing agents, etc. However, the above dilemma has not basically been solved, and under the present situation, no toner is obtained, which is fixable from a very low temperature and yet excellent in off-set resistance and blocking resistance with a broad fixing temperature range.
Meanwhile, low molecular weight and amorphous polyester resins and epoxy resins are recently attracting attention as materials matching to low temperature fixing. Above all, attempts to use polyester resins as a binder for toner have been known, as disclosed in U.S. Pat. Nos. 3,590,000 and 3,681,106.
According to these disclosures and the experimental investigations by the present inventors, in the toner using a polyester resin as the binder, the fixing temperature can be generally lowered than that in case of other materials, such as styrene-acrylic resins conventionally widely used in the art, but improvement of off-set resistance has not yet been achieved.
The primary thought for off-set prevention in the toner using a polyester resin as the main binder was, as represented by U.S. Pat. No. 3,681,106, to aim at improvement of off-set resistance during fixing-melt by non-linearly modifying the polymer backbone by mixing a trivalent or more polyol or polyacid with the polymer thereby to impart rubbery elastic characteristics to the resin. The present inventors have also investigated variously on these Examples and found that these non-linear modifications are indeed effective for improvement of off-set resistance, but they also suffer the disadvantage similarly as in case of crosslinking of styrene type resins that the fixing point is elevated if non-linear modification is practiced to impart sufficient off-set resistance, thus failing to make avail of the advantage of low temperature fixability possessed by the polyester type resin. Also, as different from the improvement by crosslinking in styrene type resins, application of non-linear modification or crosslinking in polyester type resins will result in increase to a great extent of an acid value or a hydroxyl value of the resin obtained under the same reaction conditions. Probably due to this effect, humidity resistance was worsened and also it was found to have deleterious influences on charge characteristics. Then, in order to avoid such a phenomenon, experiments were repeated by changing variously the reaction conditions. As the result, when, for example, the polymerization time was sufficiently elongated, the acid value and the hydroxyl value were indeed made smaller, but crosslinking is progressed so far to result in marked elevation of the fixing temperature. According to the speculations by the present inventors, such results appear to reflect the fact that, in crosslinking under certain reaction conditions, the chances of association between the functional groups at the terminals to be reacted are extremely cancelled due to the so called steric hindrance, caused by the bulky structure like thread balls of the polymer backbone in the course of forming a three-dimensional network of the backbone at the stage of condensation.
On the other hand, there is also an attempt to prevent off-set by providing a "weak crosslinked structure" formed with metallic ions by mixing a polyester type resin with a polyvalent metal compound, thereby changing the melt-viscoelasticity of the resin through interactions between the polymer chains. The present inventors were also interested in this technique and attempted various experimental investigations. As the result, by addition of a metal compound, its effect with respect to off-set prevention on a heat roll was indeed confirmed to be obtained. However, in most of metal compounds, a considerably large amount of metal compound, namely 4 to 25 parts by weight based on 100 parts by weight of the binder resin, is required to be added in order to extract sufficiently its effect. For this reason, similarly as in case of a large amount of inorganic fillers added, probably due to increase in heat content, there appears markedly the defect of elevated fixing point. Also, due to incorporation of inorganic compounds lower in specific resistivity as compared with the polymer, the charge ability of the toner is cancelled to a considerable extent, to find that the toner tends to be worsened extremely in the developing characteristics. Thus, the method of adding a metal compound, although it may be good only with respect to improvement of off-set resistance, has a number of defects as mentioned above and therefore it is very difficult to say that this method is acceptable in commercial application. In spite of its effect, the essential point may be considered to reside in how the amount of the metal compound added can be suppressed. The present inventors have made much of this point and conducted sufficient investigations by varying systematically the kinds of the metal compounds and the partners of polyester resins. For the reason which has not yet been clarified, even when the same metal compound may be added in the same quantity, the improved effect of off-set and elevation of fixing point and the influence to worsen humidity resistance were found to be changed variously depending on the partner polyester resin. Although the influence by the acid value of the polyester resins may well be recognized, when the above actions were searched for the polyesters with the same constitution having various acid values, the results obtained were too diverse to point a particular tendency in a certain direction.
As can be seen from the disclosures in Japanese Laid-open Patent Applications Nos. 94362/1981, 116041/1981, and 166651/1980, a measure to improve off-set resistance has been found and is on the way to practical application, in which a polvalent metal compound is added to cause a kind of crosslinking, gellation. However, the reaction for imparting this crosslinkage is carried out between the solids while under molten state by heating, whereby the effect depends on the chances of association during kneading, and it is very difficult to control the kind and the amount of the metal compound, the reaction conditions, etc. which are very severe, and therefore it is hardly possible to control uniformly dispersions of various materials and maintain a definite degree of crosslinking.
These problems may be considered to be attributable to various unknown factors such as, for example, what extent of crosslinkage is necessary, absence of appropriate detection means therefore, what physical properties of the toner should be changed by crosslinking, etc.
This is why no polyester type toner has been realized, which is superior as overall characteristics over the styrene-acrylic type toner hitherto practiced frequently in the art, and the excellent characteristics of a polyester type toner fall to be utilized usefully to give no toner which can be fixed at lower temperatures, is free from off-set and good in storage stability, as required strongly at present time.